dgram: support for setting socket buffer size

* setRecvBufferSize(int) and setSendBufferSize(int)
* added docs for send/receive buffer sizes
* Added options support to set buffer sizes in
  dgram.createSocket().

PR-URL: nodejs/node#13623
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/errors.js

@@ -261,13 +261,16 @@ E('ERR_SERVER_ALREADY_LISTEN',
 E('ERR_SOCKET_ALREADY_BOUND', 'Socket is already bound');
 E('ERR_SOCKET_BAD_BUFFER_SIZE', 'Buffer size must be a positive integer');
 E('ERR_SOCKET_BAD_PORT', 'Port should be > 0 and < 65536');
+E('ERR_SOCKET_BAD_BUFFER_SIZE', 'Buffer size must be a positive integer');
 E('ERR_SOCKET_BAD_TYPE',
   'Bad socket type specified. Valid types are: udp4, udp6');
 E('ERR_SOCKET_BUFFER_SIZE',
   (reason) => `Could not get or set buffer size: ${reason}`);
 E('ERR_SOCKET_CANNOT_SEND', 'Unable to send data');
 E('ERR_SOCKET_CLOSED', 'Socket is closed');
 E('ERR_SOCKET_DGRAM_NOT_RUNNING', 'Not running');
+E('ERR_SOCKET_BUFFER_SIZE',
+  (reason) => `Could not get or set buffer size: ${reason}`);
 E('ERR_STDERR_CLOSE', 'process.stderr cannot be closed');
 E('ERR_STDOUT_CLOSE', 'process.stdout cannot be closed');
 E('ERR_STREAM_WRAP', 'Stream has StringDecoder set or is in objectMode');